This invention relates generally to liquid scintillation media, and more particularly, to liquid scintillation media comprising 1,2-diphenylethane and derivatives thereof, and to methods of detecting beta-ray emissions using the novel liquid scintillation media.
Liquid scintillation counting is a well-known method for detecting and measuring the concentration of radioactive elements in a sample. It is a particularly useful technique for measuring low energy beta-emitting elements, such as .sup.3 H, .sup.14 C, .sup.35 S, and .sup.32 P. Liquid scintillation counting is accomplished by combining the radioactive sample to be analyzed with a liquid scintillation solution or media. The liquid scintillation media comprises, inter alia, an aromatic hydrocarbon scintillation solvent and a fluor. Energy from radioactive decay in the sample excites the aromatic solvent in the scintillation media which then transfers its increased energy to the scintillation fluor, or a combination of fluors. The fluor then releases the increased energy in the form of light pulses which are proportional to the amount of radioactivity in the sample. The light pulses are quantified, or counted, by conventional photomultipliers and associated circuitry, in a known liquid scintillation counter.
Well-known and commercially-used scintillation media typically comprise the following aromatic scintillation solvents: toluene, xylenes, ethylbenzenes, cumenes, pseudocumene, mesitylene, phenylcyclohexane, anisole, and dioxane containing a small portion of dissolved napahthalene.
The aforementioned scintillation solvents have many disadvantages, such as high vapor pressures and relatively low flash points making them dangerous and inconvenient to use. Moreover, these solvents are toxic and generate hazardous waste. An additional significant disadvantage is that these solvents have relatively high rates of permeation through the wall of polyethylene, polypropylene, or other plastic scintillation counting vials commonly used in the industry. Diffusion of the scintillation solvent through the wall of the vial results in an apparent error in the measurement of quenching. This error in turn leads to an error in the calculation of efficiency and, hence, in the calculation of decompositions per minute.
A more recently developed scintillation media comprises diisopropylnaphthalenes, as disclosed in U.S. Pat. No. 4,657,696 which issued to James Thomson on Apr. 14, 1987. However, diisopropylnaphthalenes have relatively long fluorescence decay times, and therefore, cannot respond quickly to a second excitation by another beta-ray until ground state is reached. As a consequence, the efficiency of this material in a scintillation media is reduced.
It is, therefore, an object of this invention to provide an improved liquid scintillation medium composition.
It is another object of this invention to provide an improved liquid scintillation medium composition which comprises a scintillation solvent having a lower vapor pressure and higher flash point than the scintillation solvents which are presently available.
It is also an object of this invention to provide an improved liquid scintillation medium composition which comprises a scintillation solvent which has lower toxicity to humans and animals and does not constitute hazardous waste.
It is a further object of this invention to provide an improved liquid scintillation medium composition which comprises a scintillation solvent which does not penetrate plastic scintillation vials, and which does not produce error in the measurement of quenching.
It is additionally an object of this invention to provide an improved liquid scintillation medium composition which comprises a scintillation solvent which has a short fluorescence decay time, and therefore can achieve higher counting efficiency, higher gross count capability, and greater quench-resistance.
It is yet a further object of this invention to provide an improved liquid scintillation medium composition which is relatively inexpensive and easy to produce.
It is also another object of this invention to provide an improved method for liquid scintillation detection and counting of beta-ray emissions.